Method of producing a color change in a chemically coupled color-changing display

ABSTRACT

A method of producing a color change in a distinct element of solid, insoluble color changing material capable of reversibly changing color by reaction with soluble reactants. Reactants at a point spaced apart from the distinct element are electrochemically generated, diffuse through an electrolyte, and react with the distinct element to cause a reversible color change.

The invention herein described was made in the course of or under acontract or subcontract thereunder with the Department of the Air Force.

This application is a division, of application Ser. No. 327,856, filedDec. 7, 1981 now U.S. Pat. No. 4,456,337.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of electrically controllabledisplays, and more particularly to the field of displays which change orswitch color such as, for example, electrochromic display devices.

2. Description of the Prior Art

There are many uses for electrically controllable display devices. Anumber of such devices have been in commercial use for some time. Thesedisplay devices include liquid crystal displays, light emitting diodedisplays, plasma displays and so on. Light emitting diode and plasmadisplay panels both suffer from the fact that they are active, lightemissive devices which require substantial power for their operation. Inaddition, it is difficult to fabricate light emitting diode displays ina manner which renders them easily distinguishable under bright ambientillumination. Liquid crystal displays suffer from the disadvantage thatthey are operative only over a limited temperature range and havesubstantially no memory within the liquid crystal material. Further, thevisibility of many liquid crystal displays decreases as the viewer movesa few degrees off axis.

Electrochromic displays have been developed which display informationthrough a change in the color of portions in the display viaelectrochemical reaction of an active material to achieve a colorchange. Generally, with a metal oxide as the active material, this colorchange is from white to a color such as blue. Because of their coloringmechanism, such displays usually require substantial power and time towrite or erase displayed information. The quantities of power requiredare undesirably large, especially for battery operation. Moreover, thetime required to change displayed information makes such materialsunacceptable for many display applications. None of these displays showsmore than a single color against a background. This limits theversatility of such displays since the variation of color of a charactercannot be used to convey additional information.

Rare-earth diphthalocyanines are known from prior publications to haveelectrochromic properties in which the color of the diphthalocyanine canchange over a period of about eight seconds upon application of apotential difference across an electrochemical cell having adiphthalocyanine film on one of the electrodes. P. N. Moskalev and I. S.Kirin, "Effect of the Electrode Potential on the Absorption Spectrum ofa Rare-Earth Diphthalocyanine Layer," Opt. i Spektrosk, 29, 414 (1970)and P. N. Moskalev and I. S. Kirin, "The Electrochromism of LanthanideDiphthalocyanines," Russian J. Phys. Chem., 46, 1019 (1972).

U.S. Pat. No. 4,184,751 of M. M. Nicholson, the inventor herein,describes the use of metal diphthalocyanine complexes as theelectrochromically active material in an electrochromic display cell.Rapid color changes in less than 50 milliseconds are achieved, thusalleviating the slow switching time previously reported for rare-earthdiphthalocyanine complexes. Power requirements are small because of thelow power switching characteristics of the display material and becausethe display exhibits an open circuit memory of from several minutes toseveral hours, depending on its construction. A multi-color, i.e., morethan one color, display is achieved through use of a range of voltagesapplied between display and counter electrodes. Color reversal ofdisplayed information and the background against which it is displayedis achieved through use of display electrodes in the background portionsof the viewing area as well as in the character segments.

Matrix display devices contain one or more arrays of many small elementsor dots of color-changing material that can be selectively activated orswitched to form virtually any alphanumeric or graphic pattern. Tocreate such patterns and erase them at will, a means must be provided toaddress each element independently without activating those in thesurrounding area. An integrated drive matrix of thin-film transistorscould be built into the display device for this purpose so that eachelement is provided, in effect, with a separate switch connecting it tothe power supply. See T. P. Brody and P. R. Malmberg, "Large ScaleIntegration of Displays Through Thin-Film Transistor Technology," Int.J. Hybrid Microelec., II, 29 (1979).

Although the use of an integrated drive matrix is an elegantgeneral-purpose approach for matrix displays, its fabrication isrelatively complicated. When possible, electronics engineers prefer touse the simpler multiplexed addressing scheme of the sort which uses twosets of parallel conductive, linearly-extending electrodes disposed atright angles. A thin layer of the display material and any associatedcomponent such as an electrolyte is disposed between the two sets ofelectrodes. When a display system is fully amenable to such addressing,a single selected dot or element can be activated by a signal appliedacross one electrode in each of the two sets of orthogonal electrodes.

But the use of multiplexed addressing with a matrix display usingelectrochromic materials can cause the loss of some of the inherentadvantages of these materials. Even though electrochromic displaysexhibit appropriate voltage threshold characteristics for goodresolution of the individual elements or dots, the advantages of apotentially outstanding feature of such displays, memory or patternretention, may be lost if the displays are multiplexed as describedabove. This is particularly true of a multicolor electrochromic systemthat provides different colors by undergoing both oxidation andreduction reactions. Memory or pattern retention is lost because ofgalvanic action between oppositely charged elements in contact with thesame electronic conductor and electrolyte. The elements discharge tointermediate electrical and color states thereby erasing theinformation. A similar problem exists in a multiplexed electrochromicdisplay containing a combination of a redox couple and a solubleswitching material. See Arellano et al., "Matrix AddressedElectrochromic Display," U.S. Pat. No. 4,146,876 (1979). The loss ofmemory problem is overcome in the Arrelano et al. approach by frequentrefreshing of the color through the use of a repetitive input signal.Unfortunately, this increases the power consumption.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the invention is concerned with a displaydevice wherein information is developed in one or more electronicallyisolated dots or elements of an insoluble display material which iscapable of reversibly changing or switching its color by reaction withsoluble oxidizing and reducing agents. An electronically isolated dot orelement is one which is situated so that it cannot physically transferelectrons directly to or from an external circuit or to or from anyother such dot or element. Such an electronically isolated dot orelement is not prevented, however, from undergoing chemical electrontransfer reactions with dissolved oxidizing or reducing agents andtransferring an equivalent number of charge-compensating ions to or fromthe electrolyte. Each isolated dot of color-changing material is adisplay element associated with or aligned with and chemically coupledto a distinct electrode crossover region in a nearby electrochemicaldrive matrix which may be addressed by direct multiplexing. The solublereactants are electrochemically generated at the drive matrix and aretransported to the correspondingly selected display element by diffusionthrough a thin layer of electrolyte.

Since the display elements are electronically isolated from each other,there can be no galvanic interaction between them. Thus, there can be nospontaneous electrochemical erasure of information. In view of thisisolation, a display device in accord with the invention has good memoryor pattern retention in the absence of an applied signal. It thus can bedriven or addressed by direct multiplexing circuitry, provided the drivematrix is capable of selectivity of a selected display element in thetransient mode.

It follows then that a matrix display in accord with the inventionshould be relatively inexpensive to manufacture due to the simplicity ofits directly multiplexible structure when compared with a relativelyexpensive integrated matrix drive structure based on thin-filmtransistors or other solid-state electronic technology.

Films of the color-changing display material used in this invention arepreferably supported on an inert, insulative substrate of a fullycompatible material. There is no need to dispose the color-changingmaterial on a conductive transparent material such as tin oxide.Furthermore, by matching the thermal expansion coefficient of thesubstrate to that of the display material, the adhesion between themshould be quite high. This factor will tend to increase the useful lifeof a display device in accord with the invention.

Since plastic can be used for the insulative substrate instead of glass,resistance to breakage can be increased.

When display material is disposed directly on an electrode, certainother deleterious effects can occur. For example, cathodic hydrogenevolution can cause a lutetium diphthalocyanine film to peel away from atin oxide electrode. If, as in the invention, the color-changingmaterial is not on an electrode surface, this problem cannot occur. Inthe present invention, the display material can be on any suitablesubstrate.

The display described in Arrelano et al., as mentioned above, lacksmemory and has to be refreshed frequently by a repetitive input signal.On the other hand, since a device in accord with the invention uses aninsoluble color-changing material rather than a soluble one, suchrefreshing is not required. Hence the average power is much lower.Furthermore, due to its pattern retention feature, information displayedin a device in accord with the invention is not lost in the event of apower failure.

The rare-earth diphthalocyanines are useful as electrochromic materialsdisposed directly on electrodes due, in part, to their relatively highsolid-state conductivities. Of course, these materials are also expectedto be well-suited for use in this invention. However, depending on thechemical kinetics, it is believed that it may be possible to use in thisinvention many other materials that can change color reversibly butwhich lack high solid-state conductivity or other properties favorableto a direct electrochromic response. This broad aspect of the inventionexists because the color change reactions therein are essentiallychemical rather than electrochemical.

It is not necessary with this invention to view the display elementsthrough a transparent semiconductive substrate made of, for example, tinoxide or to insulate portions of the drive lines from the electrolyte.Electrodes in the drive matrix can be fabricated of a much more highlyconductive metal which can be in direct contact with the electrolyte.This feature contributes to making a display device in accord with theinvention relatively easy to fabricate. When a display device is to beviewed by reflected light only, the drive matrix ordinarily will beconcealed by a body of white, porous optical background material whichis immersed in and permeated by the liquid electrolyte. If the displayis to be viewed by transmitted light, in a backlighted or projectedarrangement, the drive matrix must be at least semitransparent. Theelectrodes may then consist of, or be disposed on, strips ofsemitransparent metal mesh.

In relation to the switching speed, the problem of line resistance isless serious in this display device with its metal electrodes than it iswhen long lines of tin oxide are used. In addition, although layerseparation or thickness tolerances are important in a display device inaccord with this invention, they can be sufficiently controlled by usingstate-of-the-art screening or lamination techniques. Therefore, itshould be practical to build relatively large display panelsincorporating this invention without having to construct exceedinglysmooth plates of a material such as glass and then to secure the platesin closely spaced parallel disposition as in liquid crystal displaydevices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view in perspective of the internal elements of acolor-changing display device in accord with the invention.

FIG. 2 is a cross-section of a portion of a color-changing displaydevice in accord with the invention.

FIG. 3 is an exploded view in perspective of the internal elements of analternative embodiment of a color-changing display device in accord withthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there are shown the essential internal parts ofa chemically coupled color-changing matrix display device 10. A displaymatrix 12 comprises a plurality of coplanar, electronically isolateddots or elements of a solid, insoluble display material preferablydisposed in orthogonal rows and columns on a planar surface of aninsulating substrate 14. The display material may be any insolublecolor-changing material such as, for example, a multicolor rare-earthdiphthalocyanine, a two-color (blue and white) indigo dye or otherinsoluble dye which is capable of reversibly changing color by reactionwith soluble oxidizing and reducing agents that are electrochemicallygenerated. The substrate 14 may be of any compatible material such as,for example, a plastic, glass or alumina plate or a plastic film. Thesubstrate 14 is preferably of a material which has substantially thesame thermal expansion coefficient as the color-changing materialdisposed thereon so as to promote good adhesion thereto. The better theadhesion, the longer will be the life of the display device 10.

The remainder of the display device parts in FIG. 1 comprise drivematrix means for electrochemically generating the reactants, i.e., thesoluble oxidizing and reducing agents. The reactants interact with thecolor-changing material to alter its color. The drive matrix is disposedparallel to and spaced apart from the planar surface of the substrate 14on which the display matrix 12 is disposed.

The drive matrix includes a first linear array or set of generatorelectrodes 16 and a second linear array or set of counter electrodes 18.Each electrode is a relatively long and narrow, isolated conductive unitdisposed parallel to the other electrodes of its array. The electrodesof each linear array are preferably disposed at right angles to ororthogonal to the electrodes of the other linear array in a distinctelectrode plane spaced apart from and parallel to the other electrodeplane. Both electrode planes are preferably parallel to the planarsurface of the substrate 14 on which the display matrix 12 is disposed.The generator electrode plane is closer to the display matrix 12inasmuch as it is interposed between the counter electrode plane and thedisplay matrix 12.

Each intersection of an individual generator electrode 16 and anindividual counter electrode 18 defines an electrode crossover region inwhich reactants for effecting color change are to be generated when anelectrical signal of appropriate magnitude and polarity is appliedacross a selected generator electrode-counter electrode combination.

In this embodiment, the substrate 14 acts as a spacer between thedisplay matrix 12 and the generator electrodes 16. The substrate andspacer 14 is porous enough to permit ready access of theelectrochemically generated reactants to the color-changing material.

The electrodes of one of the linear arrays are aligned with the rows ofdisplay elements in the display matrix 12 while the electrodes of theremaining linear array are aligned with the columns thereof. Therefore,each color-changing dot or element of the display matrix 12 is alignedwith a distinct generator electrode-counter electrode crossover region.This alignment is illustrated in FIG. 1 wherein the dashed line 20 isshown passing through the electrode intersection or crossover regiondefined by the third counter electrode 18 from the left edge and thethird generator electrode 16 from the front edge in their respectiveelectrode planes. The line 20 is shown extended to the display matrix 12where it intersects the display element situated at coordinates X=3, Y=3relative to an origin at the intersection of the left and front edges ofthe display matrix plane.

The display 10 is intended to be viewed in the direction indicated bythe arrow 22. This gives the more direct observation of the displaymatrix 12. If the display 10 is to be front-lighted, i.e., viewed byreflected light, the counter electrodes 18 can be opaque. However, for aback-lighted or projected display, the counter electrodes 18 must eitherbe transparent or semi-transparent. An open mesh structure will meet thelatter requirement. The generator electrodes 16 also are required tohave an open-mesh or similar structure so that reactants formed therecan escape and diffuse to the display matrix 12. For a back-lighted orprojected display, the generator electrodes 16 must be sufficientlytransparent for viewing by transmitted light.

Interposed between the generator electrodes 16 and the counterelectrodes 18 is a selective separator 24 which, in effect, divides theinterior of the display device into two compartments. The firstcompartment contains the generator electrodes 16 and the display matrix12 while the second compartment contains the counter electrodes 18. Theselective separator 24 prevents loss of electrochemically generatedreactant species from the compartment containing the generatorelectrodes 16 and the display matrix 12. Stated alternatively, theseparator 24 excludes or confines the electrochemically generatedreactant species away from the compartment containing the counterelectrodes 18. Thus, the generated reactants are preserved for reactionwith display material only. In addition, the separator 24 is required toconfine certain soluble chemical species to the compartment of thegenerator electrodes 16 and display matrix 12 and prevent contaminationof the counter electrodes 18 where these species could interfere withthe operation of the counter electrodes 18. Similarly, the separator 24is required to confine certain other soluble chemical species to thecompartment of the counter electrodes 18 and prevent contamination ofthe generator electrodes 16 and of the display material in the displaymatrix 12 where these other species could interfere with the operationof the generator electrodes 16 or with the operation of the displaymaterial. However, the separator 24 does permit the passage ofcurrent-carrying ions between the generator and counter electrode arrays16 and 18. A semi-permeable separator 24 made of, for example, an ionexchange resin is preferred but a retentive diffusion barrier containingelectrolyte may serve as an adequate separator 24 in some cases. An ionexchange resin exhibits selective permeability due to its ability totransport primarily cations or anions. A retentive diffusion barrierretards the undesired passage of chemical species because of itsmicroporous structure. The diffusion barrier can be of a microporouslayer of inert material fabricated by screening. Since these porouslayers are usually white, they can serve as optical backing in afront-lighted display or as a translucent light-transmitter in aback-lighted display. Alternatively, the separator 24 may be a molecularfilter having selective permeability due to its ability to transportonly chemical species smaller than a certain size. Excessive thicknessof the separator 24 will diminish pattern resolution in the drivematrix.

The generator electrodes 16 are preferably of a highly conductive, inertmaterial such as, for example, gold. The counter electrodes 18preferably include an electrochemical couple with insoluble activecomponents, such as silver-silver bromide, which will not impose specialrequirements on the separator 24. Soluble counter electrode couples suchas iodide-triiodide are not ruled out, however, if an appropriateseparator 24 is used. If both members of the counter-electrode coupleare soluble, as in the case of iodide-triiodide, the separator 24 mustbe retentive enough to exclude the more active member, such astriiodide, from the region of the display matrix 12.

The layer shown at 26 in FIG. 1 represents a body of electrolytesolution contacting the display matrix 12, the generator electrodes 16and the counter electrodes 18. The portion of the electrolyte solution26 in contact with the display matrix 12 and the generator electrodes 16initially contains a component of each of two redox couples. Asindicated above, any components of the redox couples that wouldinterfere with the operation of the counter electrodes 18 are excludedor confined away from the region of the counter electrodes 18 by theseparator 24. The initial component of one redox couple is in thereduced form while the initial component of the other redox couple is inthe oxidized form. The electrolyte solution 26 may also include an inertsupporting electrolyte. This may be a simple inorganic salt such as, forexample, potassium chloride. The initial redox couple components must becompatible with one another and with the color-changing material of thedisplay matrix 12 so that no color change or other change occurs untilan electrical signal is applied to the display.

Chemically, the operation of the display device 10 is similar to that ofindirect coulometry, a technique developed for the investigation ofredox processes in biological materials that are sterically unable toreact directly at an electrode surface. See F. M. Hawkridge and T.Kuwana, "Indirect Coulometric Titration of Biological Electron TransportComponents," Anal. Chem., 45, 1021 (1973).

When a current is passed in the drive matrix with the selected generatorelectrodes 16 as the anode and the selected counter electrodes 18 as thecathode, an oxidizing agent is formed at the surfaces of the generatorelectrodes 16. This reactant diffuses across the layer of electrolytesolution 26 from the generator electrodes 16 to the correspondinglyselected or addressed color-changing material in the display matrix 12.The oxidizing agent reacts with the color-changing material to changeits color and, in the process, is regenerated as the initial redoxcomponent in the reduced state. Thus, the soluble redox system mediates,or couples, the color-changing material in the display matrix 12 to thegenerator electrodes 16 without being consumed itself. In the displaycell 10, the anodic charge passed at the generator electrodes 16 shouldbe that required to completely convert the amount of color-changingmaterial present in the addressed elements of the display matrix 12. Oncontrolled electrolysis in the opposite direction, the component of theother redox couple generates a reducing agent which reacts with theoxidized color-changing material and brings it back to its initial colorstate. It is apparent that there is no net change in the color-changingmaterial or the reactants. Thus, the cycle should be repeatable manytimes. With some color-changing materials, if the reverse electrolysisis carried further by the passage of additional cathodic charge, thecolor-changing material may be reduced beyond its original color stateto a third or even a fourth color state. Hence, in addition to beingapplicable to two-color displays, the scheme of this invention isadaptable to the operation of multicolor displays wherein thecolor-changing material has more than two color states.

As will be apparent to those skilled in the art, the above-recitedprocess may be reversed in that the first reactant generated may be areducing agent to react with a suitable color-changing material toswitch the material from its initial color state by reduction ratherthan by oxidation. It will also be apparent that further oxidized statesmay exist to provide additional colors.

By way of example, a suitable color-changing material for a displaydevice 10 in accord with the invention is lutetium diphthalocyanine,often abbreviated LuH(Pc)₂, initially in a green color state. Theinitial soluble redox component in the reduced form may be the bromideanion, Br⁻. When a current is passed in the drive matrix with theselected generator electrodes 16 as the anode and the selected counterelectrodes 18 as the cathode, the bromide anion is oxidized at thegenerator electrodes 16 to form bromine, Br₂. The bromine reactantdiffuses across the electrolyte layer 26 to the display matrix 12 wherethe lutetium diphthalocyanine is switched from its initial green colorto a red color state by oxidation. In the process, the initial redoxcomponent, the bromide anion Br⁻, is regenerated.

The initial soluble redox component in the oxidized form may be thecolorless methyl viologen (1,1'-dimethyl-4,4'-bipyridyl) cation,abbreviated MV⁺⁺. On controlled electrolysis in the reverse direction, acurrent is passed in the drive matrix with the generator electrodes 16as the cathode and the counter electrodes 18 as the anode. The methylviologen cation is reduced at the generator electrodes 16 to form MV⁺.This reactant diffuses across the electrolyte layer 26 to the displaymatrix 12 where the lutetium diphthalocyanine is switched from the redcolor state to its initial green color state by reduction. In theprocess, the initial redox component, the colorless methyl viologencation MV⁺⁺, is regenerated. Although the MV⁺ species is stronglycolored, it is present only during the switching process. Hence, itshould not significantly alter the appearance of the display.

If the reverse electrolysis is carried further by the passage ofadditional cathodic charge, the lutetium diphthalocyanine may be furtherreduced beyond the green state to a blue form. The reaction sequencebelow illustrates the type of chemical process involved in thisadditional reduction.

At Metal Electrode (Generation of Reducing Agent)

    2MV.sup.++ +2e→2MV.sup.+

At Display Matrix Surface (Chemical Switching of Color-Changing Materialfrom Green to Blue) ##STR1##

Net Result (Indirect Electrochemical Switching) ##STR2##

In principal, only two redox couples are needed to cycle adiphthalocyanine film through all of its oxidation states, or colors.One redox couple should have an equilibrium potential more negative thanany in the color-changing material system, and the other should have anequilibrium potential more positive than any of those for thecolor-changing material. It is further desired that different states ofthe color-changing material within a given dot be capable of interactingwith one another to reach equilibrium fairly quickly after passage of aswitching charge. For example, in converting a lutetium diphthalocyaninefilm from red to green, any overdriving of the outer surface to blueshould be only temporary. From observations of lutetiumdiphthalocyanine, it is anticipated that such equilibration can occureasily across several thousand angstroms of film thickness. Standardpotentials are not yet known in this color-changing material system, butsome practical relations of color and absorption spectra to potentialare given in M. M. Nicholson and R. V. Galiardi, "Investigation ofLutetium Diphthalocyanine as an Electrochromic Display Material," FinalReport, Contract N62269-76-C-0574, C77-215/501, NADC-76283-30, May 1977,Electronics Research Center, Rockwell International, Anaheim, Calif.

Certain color conversions of lutetium diphthalocyanine have beenobserved. MV⁺⁺ has been electrochemically reduced to MV⁺ which thenreacted with this display material to switch its color to blue fromgreen. Br⁻ has been electrochemically oxidized to form Br₂ which thenreacted with this display material to switch its color from green tored. With mixtures containing both couples (MV⁺ /MV⁺⁺ and Br⁻ /Br₂),reversible switching has been observed.

The cross-sectional view of the display device 10 in FIG. 2 is expandedto show certain of its details. A single generator electrode 16', havingan open mesh structure, extends horizontally and perpendicular to theplane of the drawing. A single counter electrode 18', also having anopen mesh structure, extends vertically and parallel to the plane of thedrawing. Interposed between the a generator electrode 16' and thecounter electrode 18' is the selective separator 24.

A single, electronically isolated, distinct display element 12' ofcolor-changing material is shown disposed on the porous substrate andspacer 14 and aligned with the distinct intersection or crossover regionof generator electrode 16' and counter electrode 18'.

A front panel 28 for the display device envelope is of any suitabletransparent material such as a clear plastic or glass. A rear panel 30for the envelope may be of the same transparent material although it maybe of an opaque material if the display is to be front-lighted.

Two compartments 34 and 36 containing the body of electrolyte 26 areshown in FIG. 2. The compartment 34, shown to the left of the separator24, includes the generator electrode 16', the substrate and spacer 14and the display element 12'. The compartment 34 contains that portion ofthe body of electrolyte 26 having the redox components therein which areneeded to react at the generator electrode. The compartment 36, shown tothe right of the separator 24, includes the counter electrode 18'. Thecompartment 36 contains that portion of the body of electrolyte 26 fromwhich redox components are excluded unless some of them happen to becommon to the counter electrode system. For example, a component such asbromide ion can be one of the main redox components, so that

    2Br.sup.- →Br.sub.2 +2e

at the generator electrode. Sometimes the same component can be part ofthe counter electrode system:

    AgBr+e→Ag+Br.sup.-.

In this case, one can use a separator 24 which is permeable to bromideion.

The compartments 34 and 36 are shown in FIG. 2 to have substantial sizefor the purpose of providing an excess of reactants. Longer device lifeis thereby provided in the event of gradual depletion of the reactantswhen the display device 10 is put in service. Where depletion is not afactor, the display device 10 can be made more compact by making thecompartments 34 and 36 smaller.

In the embodiment of FIGS. 1 and 2, the substrate and spacer 14supporting display element 12' controls the displacement between displayelement 12' and generator electrode 16'. The spacer 14 may betransparent, translucent or, when it is used as optical backing in afront-lighted display, white. It is necessary that the spacer 14 haverelatively high porosity. Electrolyte 26 fills the spacer pores whichmust be large enough to permit virtually unobstructed passage of thesoluble reactants.

The displacement or distance between the display element 12' and theportion of the generator electrode 16' in the crossover region ofgenerator and counter electrodes 16' and 18' is very small. This isnecessary for rapid switching of color, since a reactant must travel bydiffusion from its generation site across a layer of the electrolyte 26to display element 12'. For example, if the reactant has a diffusioncoefficient of 1×10⁻⁵ cm² /sec in the liquid phase, and a switching timeof one hundred milliseconds is desired, the distance between thegenerator electrode and the surface of display element 12' should beapproximately fourteen microns. This estimate is made from therelationship Δt=(ΔX)² /2D where Δt is the transport time across a layerof thickness ΔX and D is the diffusion coefficient. With a five-micronseparation distance, the response time would be reduced to approximatelytwelve milliseconds.

On the other hand, the distance from the intersection or crossoverregion of generator and counter electrodes 16' and 18' to the displayelements adjacent to display element 12' in display matrix 12 ispreferably sufficiently great that diffusion of reactants to theseadjacent display elements from the selected electrode crossover regionis insufficient to create a visible effect. That is to say that thelateral spacing between adjacent display elements is preferably largecompared to ΔX.

The electrolyte properties and the spacing between the generatorelectrodes 16 and the counter electrodes 18 in the drive matrix shouldbe chosen to give good resolution, i.e., to generate reactant only atthe selected intersections. This condition is approached by making thatportion of the drive matrix between the generator electrodes 16 and thecounter electrodes 18 relatively thin and of relatively highresistivity. Resolution is improved further if the separator 24 is amembrane having pores extending perpendicular to the membrane surface sothat the effective resistivity of the electrolyte-membrane layer isanisotropic. A threshold voltage in the electrochemical current-voltagecharacteristic at the generator or counter electrode surface is alsoconducive to good resolution.

It is preferable to address a selected display element with a currentpulse, rather than a voltage pulse, since it is the amount of chargepassed in generating a given amount of reactant which is most closelyrelated to the amount of color-changing material to be switched.However, a voltage pulse of suitably controlled amplitude and durationmay also be used.

Shown at 32 is a support for the multilayered central structure of thedisplay device 10 comprising the display matrix 12 and the drive matrix.The support 32 is preferably porous. It is also preferably transparentif the display device 10 is back-lighted. It may be discontinuous, vis,fabricated as a plurality of small spacer pads distributed over thestructure.

Although it is important to control the various thicknesses in themultilayer device structure according to the invention, this control isnot as difficult to achieve as in the fabrication of liquid crystaldisplay devices wherein relatively large rigid plates must be positionedclose together. The layer thicknesses in the present device can beachieved by screening or lamination techniques.

The alternative embodiment of a chemically coupled color-changingdisplay device shown in FIG. 3 is similar to the embodiment shown inFIGS. 1 and 2. However, in FIG. 3, the insulating substrate 14 on whichthe display matrix 12 is disposed is on the exterior side of the displayelements rather than on the interior side. In this case, the substrate14 must be transparent while it need not be porous. In fact, the displaymatrix 12 of FIG. 3 is preferably formed directly on the interior sideof a panel which forms part of the wall or envelope for the entiredisplay device 10, thus reducing the number of parts required. In adesign for this alternative embodiment, the storage capacity of theelectrolyte compartment which includes the display matrix 12 will belimited by needs for rapid switching and adequate resolution. That is tosay, the distance between the generator electrodes 16 and the displaymatrix 12 is limited by these considerations.

While the invention has been described with respect to the preferredembodiments thereof, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention. For example,although the primary use of the chemically coupled drive conceptdescribed above is expected to be for matrix displays, the sameswitching approach can also be applied to alphanumeric and graphicdisplay devices wherein multiplexed addressing is not required.

What is claimed is:
 1. A method of producing a color change in adistinct element of solid, insoluble color-changing material whereinsaid material is capable of reversibly changing color by reaction withsoluble reactants, the method comprising the steps of:electrochemicallygenerating said reactants at a point spaced apart from said distinctelement; and disposing said distinct element to receive said reactantsfrom said point by diffusion through an electrolyte.
 2. The method ofclaim 1, wherein said soluble reactants are soluble oxidizing andreducing agents.
 3. The method of claim 2, wherein said display materialis an electrochromic material.
 4. The method of claim 3, wherein saidelectrochromic material is a rare-earth dipthalocyanine or adipthalocyanine of yttrium or scandium.
 5. The method of claim 4,wherein said electrochromic material is lutetium dipthalocyanine.
 6. Themethod of claim 1, and further comprising:providing a generatorelectrode and a counter electrode; and interposing selective separatormeans between said counter electrode for preventing loss ofelectrochemically generated soluble reactants from a compartmentcontaining said generator electrode and said element of color-changingmaterial while permitting passage of current-carrying ions between saidgenerator and counter electrodes.
 7. The method of claim 6, and furthercomprising:contacting said element of color-changing material, saidgenerator electrode and said counter electrode with a body ofelectrolyte, wherein said electrolyte initially includes, confined awayfrom said counter electrode, a component of each of two redox coupleshaving one of the two initial components in the reduced form and theother in oxidized form, and wherein the components of said redox couplescan be electrochemically converted to produce soluble reactants forreversibly changing the color of said element of color-changingmaterial.